Chemical-mechanical planarization ("CMP") processes remove materials from the surface layer of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is exposed to an abrasive medium under controlled chemical, pressure, velocity, and temperature conditions. The abrasive medium has abrasive particles that abrade the surface of the wafer, and chemicals that oxidize and/or etch the surface of the wafer. Thus, when relative motion is imparted between the wafer and the abrasive medium, material is removed from the surface of the wafer.
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. The platen 20 has a surface 22 to which an under-pad 25 is attached, and the polishing pad 40 is positioned on the under-pad 25. The primary function of the under-pad 25 is to provide a compressible, resilient medium to equalize the pressure between the wafer 12 and the polishing pad 40 across the face of the wafer 12. The under-pad 25 also protects the platen 20 from caustic chemicals in the slurry 44 and from abrasive particles in both the polishing pad 40 and the slurry 44. A drive assembly 26 rotates the platen 20 as indicated by arrow "A" and/or reciprocates the platen back and forth as indicated by arrow "B". The motion of the platen 20 is imparted to the pad 40 because the polishing pad 40 frictionally engages the under-pad 25. The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows "C" and "D", respectively.
In the operation of the conventional planarizer 10, the wafer 12 is positioned face-downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 12 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 12.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer because it is important to accurately focus circuit patterns on the wafer. As the density of integrated circuits increases, current lithographic techniques must accurately focus the critical dimensions of photo-patterns to within a tolerance of approximately 0.10-0.5 .mu.m. Focusing the photo-patterns to such small tolerances, however, is very difficult when the distance between the emission source and the surface of the wafer varies because the surface of the wafer is not uniformly planar. In fact, when the surface of the wafer is not uniformly planar, several devices on the wafer may be defective. Thus, CMP processes must create a highly uniform, planar surface.
The surface of the wafer, however, may not be uniformly planar because the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate") often varies from one area on the wafer to another. The polishing rate is a function of several factors, one of which is the temperature at the interface between the polishing pad 40 and the wafer 12. The temperature at the pad-wafer interface typically varies from one area on the pad to another for several reasons, some of which are as follows: (1) the surface contact rate between the polishing pad and the wafer generally varies positionally from one area of the polishing pad to another; (2) high points on the planarizing surface of the polishing pad have a higher temperature than other areas on the pad because the wafer contacts such high points with more pressure; (3) the abrasiveness of the pad may vary from one area on the pad to another; and (4) the cooling/heating rate of the pad varies from one area of the pad to another. Although the above-listed factors can be adjusted, altering these parameters to control the pad-wafer interface temperature may adversely impact the polishing rate or uniformity of the finished surface of the wafer.
One desirable solution to control the pad-wafer interface temperature is to adjust the temperature of the platen to heat or cool the polishing pad as needed. Controlling the polishing pad temperature with the platen, however, is difficult because the under-pad substantially prevents heat transfer between the platen and the pad. To date, heat transfer properties have been a low priority for under-pads; instead, the properties of compressibility and resiliency have influenced the development of under-pads. Under-pads must be sufficiently compressible to compensate for wafer bow and thickness variations, and they must be sufficiently resilient to resist wear. Conventional under-pads are accordingly made from a compressible matrix material and reinforcement fibers of glass, nylon or other non-conductive materials. Although the glass or non-metal fibers control the resiliency and compressibility of under-pads, they are thermal insulators that prevent heat transfer between the polishing pad and the platen. Thus, conventional under-pads make it difficult to use the platen to control the regional temperature variances across the surface of the polishing pad.
In light of the problems with conventional under-pads, it would be desirable to develop a thermally conductive under-pad that has appropriate compressibility and resiliency characteristics.